Many situations can be envisioned in which it is desirable for government bodies such as law enforcement agencies to prevent the use of cellular telephones within a certain geographical area. For example, during a situation of civil unrest such as riots, it may be desirable for the police or other similar government bodies to prevent the function of cellular telephones within a certain section of a city.
Other cases where it may also be desirable to prevent the function of cellular networks in a defined geographical area might be, for example, before a police raid on a building. Another example of a problem area which could be addressed by area-selective prevention of cellular telephony is cellular telephones which have been smuggled into prisons, which is beginning to constitute a major problem.
Traditionally, the prevention of cellular telephony within an area has been approached much in the same manner as traditional electronic warfare. This means that traditional solutions have been used which are mainly aimed at transmitting a jamming signal, usually noise, on the relevant frequencies, so as to “drown” the information in noise, and to thus create a signal-to-noise ratio which prevents the user terminals in the targeted area from detecting the “payload” signal.
A problem with the technique described above, i.e. so called “brute force” jamming, is that this is a method which consumes a great deal of energy, thus making the equipment heavy and not very portable. In addition, since noise signals are transmitted with a relatively high output power at well known frequencies, it can be quite easy for a skilled user to detect that the system is being tampered with.
Document FR 2 858 742 describes a procedure and a device for tampering with cellular communications which addresses some of the problems described above. However, the device appears to need a great deal of processor power, since it relies on a rather complex algorithm, and tries to identify certain signals on a more or less constant basis.